text:
How to estimate the approximate heading between two coordinates?
I'm having some issues regarding a navigation exam question.

Place A 57N 176(deg)W and place B 60N 177(deg)E. Calculate the difference in longitude and approximate heading from B to A.

The answer key says the difference in longitude is 7 deg (which I was able to calculate) and approximate heading 130 deg which I don't know how to calculate

@Bianfable I see, the website seems handy and I'm assuming you calculate the bearing using the formula given in the website? as the book I'm using hasn't mentioned anything about calculating headings in the chapter

Are you allowed to use a chart?

Usually in these kind of questions there is really nothing to calculate, they only test if you have understood the concept. So after figuring that you will cross from Eastern to Western hemisphere figure out towards which quadrant you are moving to and what is “more” and what is “less” and then choose the appropriate answer.

This is the type of thing that can be estimated using a flight computer, and I don't mean a digital computer, but a glorified slide rule.

@busdriver, good point.  If it's multiple choice you may be able to eliminate 3 out of 4 if you know you need to go southeast-ish.

If you want a simple way to approximate the answer - consider where it happens
At ~60 degrees, each "latitude degree" is still the same distance unit as everywhere
While "longitude degree" would only be of length $cos(\approx 60^\circ) \approx \frac{1}{2}$
In the task you need to move 3 degrees south and 7 degrees east - roughly twice more of roughly twice shorter units
So, roughly, you need to travel $45^\circ$ south-east, which is heading $135$
Considering that our whole travel is to the south of latitude 60, and since we are traveling slightly more than twice to the east (7:3 not 6:3) - you can adjust slightly more to the east, but specific adjustment is a bit to your whim.
$130$ I guess is just a round number in the neighborhood, making it more of an obvious choice than the others

Usually, questions asking you for a heading from A to B present you with a chart excerpt, or the exam requires you to have a specific sectional to hand - in my experience at least.
Having said that, A little bit of maths (or programming, if you're so inclined) can solve this problem. The main details of the calculation (in both Javascript and as Excel formulas) is detailed on https://www.movable-type.co.uk/scripts/latlong.html

This formula is for the initial bearing (sometimes referred to as forward azimuth) which if followed in a straight line along a great-circle arc will take you from the start point to the end point:1
Formula:   θ = atan2( sin Δλ ⋅ cos φ2 , cos φ1 ⋅ sin φ2 − sin φ1 ⋅ cos φ2 ⋅ cos Δλ )
where:  φ1,λ1 is the start point, φ2,λ2 the end point (Δλ is the difference in longitude)

And then the final bearing

For final bearing, simply take the initial bearing from the end point to the start point and reverse it (using θ = (θ+180) % 360).

* Note: Your inputs in degrees need to be converted to angles in radians for the formula!
If you took the average of initial and final bearing it gives you an answer of 129.32 which is pretty close to your book answer of 130 degrees.
Javascript code used to test this out:
const lat1 = 60
const lon1 = 177
const lat2 = 57
const lon2 = -176

//φ is latitude, λ is longitude
const λ1 = lon1 * Math.PI/180
const λ2 = lon2 * Math.PI/180
const φ1 = lat1 * Math.PI/180
const φ2 = lat2 * Math.PI/180

const y = Math.sin(λ2-λ1) * Math.cos(φ2);
const x = Math.cos(φ1)*Math.sin(φ2) -
          Math.sin(φ1)*Math.cos(φ2)*Math.cos(λ2-λ1);
const θ = Math.atan2(y, x);
const initialbrng = (θ*180/Math.PI + 360) % 360; // in degrees

const y2 = Math.sin(λ1-λ2) * Math.cos(φ1);
const x2 = Math.cos(φ2)*Math.sin(φ1) -
          Math.sin(φ2)*Math.cos(φ1)*Math.cos(λ1-λ2);
const θ2 = Math.atan2(y2, x2);
const finalbrng = ((θ2*180/Math.PI + 360) + 180) % 360; // in degrees

console.log("initial vs final", initialbrng, finalbrng)
console.log("average heading", (initialbrng + finalbrng)/2)

You can see this in action here: https://jsfiddle.net/0tgchsp6/

Just wondering, are pilots expected to remember such precise math for the exam? I mean, if they only asked for an approximate bearing, then aren't you allowed to calculate it approximately, with simplish math?

@IgorG, not in the US for any exam I have taken.  For this reason I favor the other answer - simple is best.

You are going FROM 57N TO 60N so you are going 3 degrees North.
You are going FROM 176W to 177E. Note the change from W to E which means you are crossing the dateline and have to remember to add or subtract 180 degrees. (There is no 180W or 180E meridian because they start at zero not 1)
To get from 176W to 177E, one degree at a time, you go West, stepping one degree at a time through 177W, 178W, 179W, 179E, 178E, to 177E, a difference of 6 degrees longitude on a heading that is two parts West for each part North or WNW (or 292.5 degrees).

This is wrong! The difference in longitude is 7 degrees because there is 180W and 180E, they are just the same.

Just do the subtraction with appropriate signs: 176°W = -176°, 177°E = +177°. Going from 176°W to 177°E is moving 177 - (-176) = +353°, meaning 353°E or 7°W, depending on which direction you fly around Earth.


text:
What is the difference between RTCA and EUROCAE standardisation?
We know that RTCA and EUROCAE working to develop standards for international civil aviation (air surveillance systems)
My questions are:

Do RTCA and EUROCAE work together for air surveillance standards or RTCA  for airborne  equipment like Mode S transponder and EUROCAE for ground station like ADS-B GS/MSSR-S ?

What is the EUROCAE standards documents for the MSSR-S Radar?

Yes, EUROCAE and RTCA do work closely together when there is an interest of both European and American industries to standardise something in a certain area.
Typically this would be the case for airborne equipment such as transponders.
Specifically in the surveillance domain there are these combined  Working Groups (EUROCAE) / Special Committee's (RTCA):

Mode-S - WG49 & SC-209
ADS-B - WG-51 & SC-186
Traffic Alert and Collision Avoidance Systems (TCAS) - WG-75 & SC-147

The EUROCAE Mode-S standard is found in ED-73F (MOPS for Secondary Surveillance Radar Mode S Transponders), the RTCA equivalent is DO-181F. These two documents have basically the same content.
The ground element of Mode-S (i.e. a MSSR-S radar) is not specified in an EUROCAE standard, but instead it is a standard published by Eurocontrol:
EUROCONTROL Specification for
European Mode S Station (EMS)

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Why do naval jet aircraft need to have strengthened undercarriages?
I've read several times that the navy versions of jet aircraft need to have a strengthened undercarriage. Here is one example, and another.
I've always just automatically assumed this was needed because aircraft landings are "rough". That is, the aircraft smacks down hard on the carrier deck (or so went my assumption).
Now I find myself questioning this. Naval jets land by catching a wire that brakes them hard. They may also takeoff with a catapult, which is some running device that pulls the nose gear forward at high g's.
So, what is the real reason that naval versions of jets need a strengthened undercarriage?

I am pretty sure this was explained somewhere on this site, including picture of the aircraft skin (of E-2C or something like that) buckling under the load on touch-down before even catching the wire, but the search does not seem to work for me lately.

@JanHudec - I addressed this question here: http://aviation.stackexchange.com/a/25090/7394
"An F/A-18 touches down around 720 fpm (12 ft/s). It's rated to twice that. CTOL fighters typically do about half that. I believe airliners average under 200 fpm (3 ft/s)."

The landing on carrier is indeed hard. The reason is not the deceleration (which is handled by the hook), but the touch-down. Since the deck is short, the wires can't be spaced very far apart, so the aircraft must touch down very precisely. Since the precision is better at steeper angle, the aircraft landing on carrier do not flare. At all. So they hit the deck at more than twice the vertical speed compared to typical landing on decent runway.

They also land with higher speed, enough to go around on a "bolter", powering up as as soon as they hit without having to wait for spool up.  Carrier landings are indeed a lot harder than normal ones.

@Simon, as far as I could find, airliners normally touch down with vertical speed below 300 ft/min (and I suppose it is similar with fighters as the speeds are similar too) while given the speed and approximately normal glide-slope, my guess would be the carrier landing would be at 600–800 ft/min. But it's just a guess.

@JanHudec you're pretty much dead right - F-18's typically land at 700fpm, and are rated up to 1,500. Airliners will typically aim to touchdown at <300fpm and most (/all?) have a maximum of somewhere around 600-700fpm before they need a "hard landing" inspection

@Simon To be very clear, they in fact unconditionally power up to full throttle as a matter of procedure as soon as they hit the deck.  If they catch the wire then it will stop them no matter what.  If they don't catch the wire then they're already roaring at full power for a touch-and-go.

Worth noting that vertical speed can also be unpredictable - the carrier deck is not fixed and will have motion of its own. It seems probable that one unfortunately-timed swell could substantially increase that relative velocity...

I'm not sure why "Naval jets land by catching a wire that brakes them hard. They may also takeoff with a catapult, which is some running device that pulls the nose gear forward at high g's." would make you question your assumption that carrier "landings are 'rough'".
In addition to Jan Hudec's description of the landing process, your statement about takeoffs is reasonably accurate, as well. According to Wiki, the C-13-1 catapult (used on many Nimitz class carriers) can shoot 80,000 lbs to 140 knots in 310 feet generating 2.81g with a total force of 225,140 lb (Thanks reirab!). All that stress goes through the nose gear.
Between the launch and the landing, there are considerably higher forces on the undercarriage of the plane, thus it needs to be considerably stronger than that of an equivalent land-based aircraft.

'g's are a measure of acceleration. 1 g = 9.8 m/s^2 = 32 ft/s^2 1 knot = 1.688 ft/s, so 140 kt = 236.3 ft/s. So, we have 310 ft = (1/2) a t^2 and a t = 236.3 ft/s. This gives us t = 2.63 s and a = 90 ft/s^2, which is 2.81 'g's. Accelerating 80,000 lb at 2.81 g requires 2.81 * 80,000 = 225,140 lb of force.

As I said, @reirab, I'm sure someone can calculate the G-force. :)  Thanks!

@FreeMan, better than that - I was once involved in measuring it. The problem we had was actually engine mounting points cracking, particularly on aircraft used for training. We didn't know g levels to expect, so we instrumented one plane and an instructor flew a sequence of increasingly heavy landings, simulating a "worst case" student. Based on that, we set up the system to record up to 30g maximum. The first set of data from real student pilots was fairly useless, because about half the data points were off the scale. On bad hair day, it's not so much "landing" as "controlled crashing".

@alephzero How in the world were the students walking away from that kind of g-force?

Those were the accelerations on the "solid" structure of the plane when the wheels hit the deck, not the same as the accelerations at the pilot's seat. The plane will pivot around the wheels, reducing the acceleration the pilot feels. But they are a measure of what the airframe has to withstand. Incidentally, after the instructor flew a landing that clocked at 10g, his comment was "I'm not being paid extra to take any more of this - that's as much data as you are going to get from me!"

The lesson being that the back-of-the-envelope can tell you pretty accurately the average force involved in flinging something into the air or bringing it to a halt. Whether it snaps or not is more critically dependent on the max force (or perhaps the max deformation) :-)

@SteveJessop alephzero was talking about vertical force on landing. My calculation was for the horizontal force applied by the catapult. That said, you're still, of course, absolutely correct about the need to design for maximum expected forces rather than just average ones. According to p12 of this paper, the shocks on an F/A-18 compress up to 12.3 inches. If the acceleration were constant (it isn't,) going from 1,500 fpm to 0 over 12.3" would require an acceleration of 305 ft/s^2 = 9.53 g.

It's the first reason you list: the aircraft hit the deck hard. It's not just the undercarriage; the whole airframe has to be ruggedized to withstand the greater shock of carrier landings.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why is there typically a large gap between the first flight and mass production of aircraft?
Here are some examples I've found on Wikipedia:
Me-262: first flight 1942 July, introduced 1944 April.
MiG-19: first flight 1953 Sep, introduced 1955 March.
Dassault Mirage III: first flight 1956 November, introduced 1961.
F-4 Phantom: first flight 1958 May, introduced 1960 December.
This shows a two or three or even five year gap between the first flight and the entering into mass production.
Why such a long gap? What goes on during those years? To me it seems like once the first flight is done, it "works" and the only thing left is some finishing touches. Is it possibly more a matter of setting up a factory?

Cars are very simple in comparison to aircraft, yet there can often be 18-24 months of testing between the first roll-out of a pre-production car and the first production model rolling off the line. There is an incredible amount of testing necessary to ensure that all the theoretical guesses that went into the design actually work in practice.

Also, 21 months to get the first jet engined aircraft ever flown from first flight to production, during a war, doesn't seem like very long at all!

@FreeMan Depends on how you look at it. I'm sure many in the Luftwaffe felt like it was an eternity, precisely because it was during a war (when they needed everything yesterday). I chose the Me-262 because, despite being an early combat jet, it seems to fit the pattern.

You don't want an aircraft that just 'works'- rather, you want an aircraft that satisfies your needs. The first flight is usually made by a prototype and does just that- it flies and pretty much nothing else.
Only once you start the flight tests do you understand the airframe limitations and whether the aircraft meets the requirements; usually, a number of modifications are needed. Also, the first flights are rarely, if ever in production configuration. In most of the cases, the required systems are fitted later. In general, most of the combat aircraft bring about substantial improvement over its predecessors- in fact it is a requirement, as the old ones can be used with small improvements at a much lesser cost.
Me 262 was really a revolutionary product and the use of jet engines caused a number of delays; in fact the prototypes were flight tested with piston engines in 1941. The first test flight happened with piston engine- just as well because both the jets flamed out soon after takeoff. By the time the second and third prototypes, the engines had been changed. However, these engines (Jumo 004) were larger, requiring larger nacelles, which in turn required a bigger vertical stabilizer (the landing gear was changed later). Pre-production of Me 262 had started in 1943 (and went nowhere due to engine troubles) when it was decided to change it into a bomber, further increasing the development period. By the time it took the field, it was in too few numbers to do anything.
Another example is the Mirage III. There were significant differences between the aircraft in first flight and the production one. The prototype never reached the required Mach 2 in level flight.The first one to do was the pre-produciotn Mirage IIIA with an engine with more than 30% extra thrust, lengthened fuselage for radar etc. Even this version was not combat capable- that was the Mirage IIIC.
The design is not frozen till the flight testing is over and the production plans can only be finalized after that- you never know what'll change (for example the hugely successful Su-27 underwent a change in wing design midway through testing). The delays due to setting up of production comes later and is normally quite less for countries/companies having experience. It is the design freeze that takes up more time during test flights.

Only once you start the flight tests do you understand the airframe limitations and whether the aircraft meets the requirements; usually, a number of modifications are needed. Can you elaborate on this more? My understanding is that wind tunnels can be run at any speed with the model at any angle of attack, so what airframe limitations are only discoverable in actual flight?

@DrZ214 I'm talking about the aircraft performance (speed, response etc.) in total. Though the wind tunnels and simulations have come a long way, still there is a (albeit reducing) gap between the ground testing and flight. One example is the first flight of F-16, which ended in disaster because the simulator didn't accurately portray the control stick forces. Modifications are also required due to other reasons- user inputs, for example; As any changes have to be tested, it takes time.

Was that F-16 flight using the pressure sensitive stick or had they already scrapped that idea?

@aeroalias the first flight of F-16, which ended in disaster Well according to wikipedia, the pilot ejected safely during that first flight. What is your definition of disaster? Just saying, many first flights end in crashes but ejection seats prevent a real disaster as far as I think of the word. Just my 2 cents.

@DrZ214 Sorry, I missed the word 'nearly'. Actually, the pilot actually landed the aircraft safely. The liftoff was accidental during high speed taxi trials, but the aircraft was landed safely by the pilot. Sorry about the mistake again.

@TomMcW The control stick of the F-16 had to be modified during the test program.  The first design, that which was implemented on the early prototypes, responded primarily to force and did not displace normally as a control stick would.  Eventually, more displacement was added in to make the control stick feel more normal, however, it still primarily measures the force input by the pilot.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Is a golf ball surface a good idea for wings or fuselage?
I searched for an infamous golf ball question on this site but did not find any, so I guess it's time for one.
Would a dimpled surface like a golf ball somehow improve the aerodynamics of an airplane? (there are many planes for many purposes, so let's just go with my favorite: large commerical passenger planes, Mach 0.85 cruise, several hundred tons loaded weight).
They actually tried this for a car on myth busters and they claimed an 11% improvement in miles-per-gallon, so you can't say the question has no merit. Unfortunately, they did not try it for an airplane.

Related: Aerodynamic Analysis of Dimple Effect on
Aircraft Wing

An off topic alternative is a vacuum system to "suck" air from the boundary layer close to the wings. It's been investigated several times even on supersonic aircraft. It has a tendency to clog up the pores, but it may be of interest: https://en.wikipedia.org/wiki/Boundary_layer_suction

At first I thought "What kind of question is this?", but after looking into it, it is a reasonable thing to ask. It looks as though such dimples would have a similar effect to vortex generators, so it follows that dimples might be good in some places but not everywhere. I look forward to seeing an informed answer to this question.

@Ben I have those same suspicions. Right now my best guesses for dimple locations are the rear half of the wing, or maybe just the underside of the rear half. For lifting bodies, who knows?

Related: http://physics.stackexchange.com/questions/109395/why-arent-airplanes-like-golf-balls

Golf ball has dimples on its surface because we want a turbulent boundary layer. Golf ball is a bluff body (i.e. the drag is dominated by pressure drag). Hence, the drag on a sphere is dominated by the separation on the rear face. If we could minimize that, the drag would be reduced.
The following figure shows the variation of drag coeffecient of a sphere with Reynolds number.

Image from wfis.uni.lodz.pl, taken from (Hoerner 1965)
As can be seen, the drag reduces at the critical Reynolds number, which is the point where separation takes place. Because of separation, the wake in the rear of the sphere (or ball) is reduced, which reduces drag. So, if the boundary layer of a sphere can be made turbulent at a lower Reynolds number (by some means), then the drag should also go down at that Reynolds number, as a result of the reduced wake. This can be seen in the following picture, where the flow has been made turbulent using a trip wire.

Flow over a sphere: (a) Reynolds number = 15,000; (b) Reynolds number = 30,000, with trip wire. Image from princeton.edu
That is the purpose of having dimples in a golf ball- to reduce drag by inducing turbulence and delaying separation (this can be achieved by increasing the Reynolds number; but we are limited by the ball speed). The resulting wake is much smaller due to the delayed separation.
In case of a streamlined shape (like aircraft wing), the propotion of pressure drag is small, as can be seen below.

Image from pilotfriend.com
In case of streamlined shapes, the pressure drag is small because the wake is small. The net result is that 'tripping' the boundary layer as in bluff bodies is not a good idea (a laminar flow causes less friction drag than a turbulent one).
The aircrafts do use similiar devices, via the vortex generator, which are used mainly to delay flow separation.

Just to relentlessly pursue the idea, are you effectively saying that the only place that is possibly viable for dimples is a place where the boundary layer has separated? (such as that small separated wake in the 3rd entry (the airfoil) of the final picture?)

@DrZ214 I'm not sure why you'd want dimples there. The dimples  cause flow to become turbulent. The golf balls have dimples all around because you don't know which direction it would fly.

@DrZ214, no, a bit ahead of that. When the flow has separated, you can't do anything about it any more. But if it is about to separate, making it turbulent will delay the separation.

" the drag reduces at the critical Reynolds number, which is the point where separation takes place" should that be "where turbulence takes place"?

Yes, the question has merit!
Short answer
It depends on the size and speed of the airplane. An airliner is too large and flies too fast to benefit from a dimpled surface. Dimpling the surface would actually increase drag. But some "dimpling" helps at the scale of model aircraft.
Explanation
It all comes down to the flow condition at the point where flow separation occurs. In calm air, every boundary layer starts as a laminar boundary layer. Since the energy transfer across the stratified flow in the laminar boundary layer is reduced to shear, the molecules close to the wall will lose speed quickly, so that even at a modest pressure rise downstream separation occurs quickly.
Inside the laminar boundary layer, small disturbances become less and less damped the higher the local Reynolds number becomes, and at a Reynolds number of around 400,000 in unaccelerated flow some frequencies become unstable (see Tollmien-Schlichting waves) and will eventually create so much cross movement that the boundary layer becomes turbulent. Now parcels of air which flow at high speed in the outer part of the boundary layer will move close to the wall and kick the slow parcels there ahead, greatly reducing the deceleration of the flow close to the wall, at the price of slowing down and expanding the whole boundary layer.
Such a turbulent boundary layer is much better in following a contour with an adverse pressure gradient since it shows less deceleration of the flow at the wall. A pressure rise is generally caused by a contracting body shape. Separation is delayed and the separation, once it occurs, is much smaller. The pressure in separated flow is lower than ambient, so rearward-facing areas with separated flow cause massive drag. Therefore, separation needs to be suppressed as long as possible to minimize drag.
If the contour of a body contracts at a local Reynolds number below that where natural transition to a turbulent boundary layer occurs, the still laminar boundary layer will cause early separation. The dimples of a golf ball help to trip the boundary layer early into its turbulent version, thus delaying separation and reducing drag.
If the local Reynolds number (which is proportional to the product of speed and body length) is higher, such that the boundary layer turns turbulent before the part of the body with the adverse pressure gradient is reached, dimpling the surface will still cause earlier transition, but will not change the separation location. Even worse, it will reduce the area of laminar flow, and the increased area with a turbulent boundary layer will cause more friction drag. The pressure fluctuations along a dimpled surface will weaken the boundary layer, and the dimpled airplane will show earlier and more separation compared to a smooth version.
Dimpling the whole surface is not necessary if the local direction of flow is known. A golf ball needs to have dimples all around so that at least some end up where they can help to trip the boundary layer, but on a wing only a small strip needs such a tripping device, called a turbulator. Some model aircraft and gliders already use them to achieve less separation, but here small dimples are less effective than a zig-zag tape. How it works merits a question of its own.

Spool of zig-zag tape (picture source)

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How to convert aircraft position from GPS to Local Tangent Plane (NED) coordinates?
Do these conversion formulas look correct? I'm writing my own local navigation script in Python, and I want to ensure that my understanding of the underlying math is solid.
Reference location
Conversion from GPS to ECEF:
$$
\begin{pmatrix}
X_{r}\\
Y_{r}\\
Z_{r}
\end{pmatrix}
=
\begin{pmatrix}
\left(N_{r}+h_{r}\right)\,\cos\left(\phi_{r}\right)\,\cos\left(\lambda_{r}\right)\\
\left(N_{r}+h_{r}\right)\,\cos\left(\phi_{r}\right)\,\sin\left(\lambda_{r}\right)\\
\left(N_{r}\left(1-e^{2}\right)+h_{r}\right)\,\sin\left(\phi_{r}\right)
\end{pmatrix}
$$
Aircraft location
Conversion from GPS to ECEF:
$$
\begin{pmatrix}
X_{p}\\
Y_{p}\\
Z_{p}
\end{pmatrix}
=
\begin{pmatrix}
\left(N_{p}+h_{p}\right)\,\cos\left(\phi_{p}\right)\,cos\left(\lambda_{p}\right)\\
\left(N_{p}+h_{p}\right)\,\cos\left(\phi_{p}\right)\,\sin\left(\lambda_{p}\right)\\
\left(N_{p}\left(1-e^{2}\right)+h_{p}\right)\,\sin\left(\phi_{p}\right)
\end{pmatrix}
$$
Aircraft-reference displacement vector:
$$
\Delta_{r_{ECEF}}
=
\begin{pmatrix}
\Delta X\\
\Delta Y\\
\Delta Z
\end{pmatrix}
=
\begin{pmatrix}
X_{p}-X_{r}\\
Y_{p}-Y_{r}\\
Z_{p}-Z_{r}
\end{pmatrix}
$$
Direction-Cosine-Matrix (DCM):
$$
\mathbf{R}\left(\phi_{r},\,\lambda_{r}\right)
=
\begin{pmatrix}
-\sin\left(\phi_{r}\right)\,\cos\left(\lambda_{r}\right) & -\sin\left(\phi_{r}\right)\,\sin\left(\lambda_{r}\right) & \cos\left(\phi_{r}\right)\\
\cos\left(\lambda_{r}\right) & -\sin\left(\lambda_{r}\right) & 0\\
-\cos\left(\phi_{r}\right)\,\cos\left(\lambda_{r}\right) & -\cos\left(\phi_{r}\right)\,\sin\left(\lambda_{r}\right) & -\sin\left(\phi_{r}\right)
\end{pmatrix}
$$
North-East-Down (NED) conversion:
$$
\begin{pmatrix}
N\\
E\\
D
\end{pmatrix}
=
\begin{pmatrix}
-\sin\left(\phi_{r}\right)\,\cos\left(\lambda_{r}\right) & -\sin\left(\phi_{r}\right)\,\sin\left(\lambda_{r}\right) & \cos\left(\phi_{r}\right)\\
\cos\left(\lambda_{r}\right) & -\sin\left(\lambda_{r}\right) & 0\\
-\cos\left(\phi_{r}\right)\,\cos\left(\lambda_{r}\right) & -\cos\left(\phi_{r}\right)\,\sin\left(\lambda_{r}\right) & -\sin\left(\phi_{r}\right)
\end{pmatrix}
\begin{pmatrix}
\Delta X\\
\Delta Y\\
\Delta Z
\end{pmatrix}
$$

Don't think this is an Aviation topic. Sure, planes use GPS, but so do cars, and hikers, and cell phones, and plenty of other things. Maybe more on-topic at edit: GIS.

@RalphJ I have a different perspective  on that; these coordinate transformations are typical for aviation system developers, whether you are working on FMS navigation implementations, radar data fusion and tracking, flight simulation (both for ATC and for flight training) and probably a few more cases. I agree it may have no direct relation to piloting an aircraft, but our scope is wider than that. Certainly it does not belong at Electrical Engineering, perhaps at GIS it would get an answer. For now I would leave it here, and if nobody answers it, I may have a dig at it myself.

@DeltaLima Yeah, GIS not EE, that seems like definitely the better fit.  But "how do I implement these formula in Python" seems pretty far outside of Aviation. The amount of coding that gets posted here in answers is almost zero.  That said, I'll leave it be & let the community weigh in to close or leave open.

If I understand it correctly, you would like to calculate the position of the aircraft in a North East Down (NED) reference frame. The origin of that reference frame is at the reference location.
The standard approach to do this uses 3 steps:

convert both positions into the same carthesian frame (usually Earth Centered, Earth Fixed, ECEF)
move the origin of the first frame to the reference point
transform to the coordinates by rotating the reference frame to the local NED orientation.

You are performing these steps in your formula, let's take a look at them one-by-one.

Your first step converts the positions of the aircraft and the reference location from geodetic latitude, longitude, height above ellipsoid ($\phi ,\lambda, h$) into the ECEF frame.
Your conversion seems correct, but for completeness sake, I will add here that you have to calculate $N_r$ and $N_p$ based on their geodetic latitude $\phi$:
$ N(\phi )={\frac {a^{2}}{\sqrt {a^{2}\cos ^{2}\phi +b^{2}\sin ^{2}\phi }}}={\frac {a}{\sqrt {1-e^{2}\sin ^{2}\phi }}}$
With:
$\begin{eqnarray}
a &=& \text{length of Earth's semi-major axis}\\
b &=& \text{length of Earth's semi-minor axis}\\
e &=& \sqrt{1-{\frac {b^{2}}{a^{2}}}} \text{the first numerical eccentricity of the ellipsoid}. \\
\end {eqnarray}
$
The second step is a simple subtraction, which seems to be correct in your approach.
The third step is the rotation of the frame to the NED attitude, which you do using the direction cosine matrix. It seems to me that there is a mistake there.
I used the following verification test:

Set the reference point at $(0,0,0)$, meaning it is the intersection of the prime meridian and the equator, at the ellipsoid surface.
In ECEF that would be $(a,0,0)$.
Suppose our aircraft is 100 m east of that location, in ECEF: $(a,100,0)$,
the difference vector being $(0,100,0)$.
calculating the east coordinate, according to your DCM would be $0\cdot\cos(\lambda_r) -100\cdot\sin(\lambda_r) = 0$, but the expected result would be 100.

Therefore, my conclusion is that your DCM is wrong.

I have this DCM matrix for conversion to the ENU (east north up) frame:
$
\begin{pmatrix}
E \\
N \\
U \end{pmatrix} =
\begin{bmatrix}
-\sin(\lambda_r) & \cos(\lambda_r) & 0 \\
-\sin(\phi_r)\cos(\lambda_r)& -\sin(\phi_{r})\sin(\lambda_{r}) & \cos(\phi_r) \\
\cos(\phi_r)\cos(\lambda_{r}) & \cos(\phi_{r})\sin(\lambda_{r}) & \sin(\phi_r)
\end{bmatrix}
\begin{bmatrix}
X_{p}-X_{r}\\
Y_{p}-Y_{r}\\
Z_{p}-Z_{r}
\end{bmatrix}
$
Rearanging the first two rows and negating the third, would yield the NED DCM matrix:
$
\begin{pmatrix}
N \\
E \\
D \end{pmatrix} =
\begin{bmatrix}
-\sin(\phi_r)\cos(\lambda_r)& -\sin(\phi_{r})\sin(\lambda_{r}) & \cos(\phi_r) \\
-\sin(\lambda_r) & \cos(\lambda_r) & 0 \\
-\cos(\phi_r)\cos(\lambda_{r}) & -\cos(\phi_{r})\sin(\lambda_{r}) & -\sin(\phi_r)
\end{bmatrix}
\begin{bmatrix}
X_{p}-X_{r}\\
Y_{p}-Y_{r}\\
Z_{p}-Z_{r}
\end{bmatrix}
$
Performing the same quick verification test yields    $-0\cdot\sin(\lambda_r) + 100\cdot\cos(\lambda_r) = 100$, and that is the expected result.
This is only proof that the result is not wrong for this simple input; I recommend doing further verification to ensure general correctness of both the formula and your implementation.


text:
How does ATC maintain separation between aircraft flying at different speeds?
How are collisions avoided when 2 aircraft are flying same altitude and same direction but have different speed characteristics.
Example a B737 and an ATR are both flying at 25000 feet. 737 will cruise at 850 Km/h whereas the ATR will cruise at 550 Km/h and the 737 is approaching from behind.
How is this type of situation handled by the pilots and ATC?
My question is specific to airliners as I have seen from websites like flightradar there are thousands of planes flying at any given moment and was wondering how this problem is handled.

FL250 for a Boeing 737 in cruise at 850 kph/460 knots GS is an unlikely value. Aircraft in the same category of speed, such as the Boeing 737/747/767/777 usually fly at higher altitudes, e.g. FL300-FL400. Slower turboprop aircraft such as the ATR72 usually fly in the ranges of FL200-FL250. During cruise, airspeed will be matched by ATC for all aircraft where the trailing aircraft might catch up to the preceding aircraft.
ATC sees the ground speed on their radar screens and can thus compare and evaluate if they need to assign speeds to maintain separation.

(Image Source: What data does ATC see on their screens? - Speed: 395 knots GS)
R: DLH123, report mach number.
A: Mach .78, DLH123
R: BER456, report mach number.
A: Mach .79, BER456
R: BER456, reduce speed to mach .78
A: Reducing mach .78, BER456

The same principle applies to aircraft in the terminal area, the lower you get, the more will aircraft have the same groundspeed when assigning the same indicated airspeed, as air density and wind approach the same values. ATC will sometimes assign a slower speed to trailing aircraft if they need to achieve the same groundspeed at different altitudes.
R: DLH123, descend to 3000ft, reduce speed to 210 knots
A: Descending to 3000ft, speed 210 knots, DLH123
R: BER456, descend to 4000ft, reduce speed to 200 knots
A: Descending to 4000ft, reducing speed to 200 knots, BER456

For aircraft that still have significantly different groundspeeds on the same airway, ATC can also vector aircraft around the preceding aircraft and put it back in front or they can issue a direct that will achieve the same effect, changing the sequence of the aircraft. A climb or descend can be coordinated with the pilots, e.g. the trailing aircraft can be asked if they are able to accept a climb to another flight level, maybe even temporarily.

Such situations usually occur when you intercepted the localizer and is coming in for a landing, where many aircrafts line up in the air one behind the other coming in to land. These situations are maintained by slowing down the aircrafts which are at the back of the first aircraft and so on. Hence you hear "(insert airline name) (insert number), you are number 2 (or number 3) for landing. Follow the company Boeing 777 on short final".
And once the aircraft at number one is landed, the second and third and so on lands one by one. Should there be a delay in the aircraft vacating the runway, the landing plane initiates a go around.
As far as planes at higher altitudes are concerned, you don't usually have such problems of one plane being exactly behind the other. The planes are slightly up or down or left or right and in this way it's easy for them to fly past one another. Hope this short answer helps.

The OP seems to inquire about speeds in the enroute or terminal environment, not on landing sequence.

Welcome. The post focuses on the approach procedure, while the question is more general, could you explain how collisions are avoided during other phases? (I think the OP is asking about how ATCOs and pilots are aware of possible collisions).


text:
What is the rationale behind the Embraer 190's control column design?
As per this picture of the Embraer 190, but also on other Aircraft, the Control column stick has a very awkward position for hands. Why is this design chosen? To me it seems that the position of the hands would be really uncomfortable as the pilots hands would be tilted at 45 degrees and not stay vertical, thus putting a lot of stress on them.

The amount of time they have one hand on the column is small.  The amount they have 2 hands on the column very small.  Why do you think it is uncomfortable?  If it was, don't you think they would have changed the design?

Have you actually tried it yourself? Sit down at a table. Rest your arms on the table in front of you, in the most comfortable position for you. Are your hands vertical? Most likely they're tilted towards each other at about 45 degrees. Now deliberately put them in a vertical position and you'll probably feel an uncomfortable twisting feeling in your forearms. So tilted is much more natural and comfortable than vertical.

I fly this type. It is not uncomfortable at all.

Actually true. Thinking about it the position of the hands is more relaxed and comfortable this way..

Actually, I think this position is more comfortable than the more common vertical layout. Consider how ergonomic computer mice are designed in a similar way, to let the hand have a 45 degree angle: http://www.computer-posture.co.uk/images/Anker-2.4G-Wireless-Vertical-Ergonomic-Optical-Mouse.gif

It looks like the one on the concorde

The main issue I would be worried about isn't the fact that the hands are at a "strange" angle, as they'd be even further from vertical on, say, a bicycle. The real problem is how the pivot point is so far under the hand position, as opposed to inbetween the hands like on other aircraft. I'm sure the pilots are used to it, but I still feel it is a suboptimal design.

Check Concorde's controls. I posted this question on pprune a few years ago. https://www.pprune.org/aviation-history-nostalgia/503342-control-wheels-v-ram-s-horns.html

I think it's quite similar to a steering wheel with your hands a bit to the upper half. You rest your hands onto the yoke with this variant, as compared to the other ones where they are vertical and inclined to slide downwards.
While I'm certainly not sure, it could be a sensory thing as well, perhaps allowing better feel for the position as you are putting less energy into your arm and tiring it out less.
I think it's a minor design detail and had it been uncomfortable/ problematic they would have got rid of it pretty quickly.
Embraer Angled Yoke
Bombadier Vertical Yoke

I flew this type several times in the sim and I think it's quite comfortable. One of the problems would be the pivot point as mentioned by CatLife. This could be tricky for the PNF. The tip could strike your knee if your legs are wide open and the pilot is using max yoke deflection. See pictures.

Another thing is the unusual inclination of the trim switches compared to traditional yokes. The first time you handle the yoke takes a few seconds to realize that the direction is not up-down. After a couple of touches most pilots are comfortable with this.

I appears that the trim switch is up/down in relation to the angle of your thumb when resting on the yoke, though. It's very natural to consider straightening the thumb to be pushing "up" and bending the thumb to be pulling "down", no matter the angle of the hand.

@FreeMan Definetly, straightening the thumb is quite natural and that's why it doesn't take more than a few touches to get familiar. I was told that a side stick was considered for the new version E2. Since this new version was fully FBW seems reasonable to have a stick like Airbus, but finally Embraer decided to keep the yoke to avoid a new type certificate. Have you heard abou it?

The hand position when gripping the Embraer handle bar is the most natural ergonomical position. For instance as stated by this site:

The natural wrist position in the field of ergonomics is the posture the wrist and hand assume when at rest. The upright position of the hand, like that of the handshake grip, is not a neutral position.

And

The natural wrist position when at rest is characterized by the following:

A straight, unbroken wrist
The hand rotated to a relaxed position (30-60 degrees)
The fingers curled and at rest
The thumb straight and relaxed


text:
How were fighter jets built in the 1950's?
As far as I know, jets did not see mass production across the world until the 1950's. So I'm interested in how they were built in that time. How many different types of factories were needed? How did those factories work? What supplies were needed and how much manpower did they need per jet?
Right now I'm only imagining two kinds, an airframe factory and an engine factory. I'm very interested in learning more and would not be surprised if there are many other types of factories that go into making a fighter jet.
For simplicity, let's exclude electronics factories like the radar, and weapons factories like guns, bullets, and missiles.

What do you mean by "mass production"?  My direct experience is a bit out of date, only going to the late 90s with the Tornado, but they were built by hand, albeit with many assemblies being delivered pre-built.  Those pre-assemblies where by and large also hand built.  I have never seen, nor heard of, "mass production" as I would recognise it, say for example in the car industry.  Can you provide an example of what you mean?

@Simon Mass production means produced on a mass scale. It does not necessarily mean produced on an assembly line. I was speaking in relative terms as well. The 50's is when jets really "took off" in production. Before that, Germany mass produced some jets in WW2, and before that it was mostly experimental jet aircraft. So it wasn't until the 50's that they were mass produced "across the world" (America, UK, USSR, etc).

Ah, I think I understand.  By mass, you mean large number of?

@Simon Yes exactly.

I think you would be wrong in saying that jets only entered 'mass production' (as you define it here) in the 1950s. Jets were produced in large numbers in the mid-to-late 1940s.

Building an airframe did not change all that much in the jet age. Assuming you are talking about all aluminum aircraft the power plant is somewhat abstracted from the equation in terms of a production line.
Here is the assembly line that Lockheed had for the early P-80 Jets (not all that different than their propellor counter parts). This picture would make it seem as though the airframe was all built in one place as you can see it in various stages in this image alone.

(source)
This is a pretty interesting article about the history of the plane and the role Kelly Johnson played in it.
Like anything in the world of production the assembly line does not change nearly as much as the tooling does. One of main costs in bringing anything to production is cutting and preparing the tooling. Assembling a jet airframe was not all to different than assembling a high performance prop plane of the era.
Things got more complex as they moved to more complicated materials (this came a bit later than your question references but none the less). By all accounts an enormous portion of the SR-71 Blackbird budget was spent developing tooling that could work with titanium.
The engine companies (Like GE/P&W etc) saw much bigger changes as they were now making and testing a completely different product. The P-80 above housed a GE/Allison J33. This would have fallen on to GE to build and like many engines prior would have simply been bought by Lockheed for the plane.
I will try and find some hard production numbers/times for this era of planes.

I'm sure large sections of the avionics were outsourced, as well. Artificial horizon, pitot tubes & IAS gauges, etc were all built by and purchased from the same companies that were building them for prop-powered aircraft. Probably seats (especial ejection seats) fall into that category, as well

Right now I'm only imagining two kinds, an airframe factory and an engine factory.

The plane's skin needs to be built, as well as all the electronics, all the cables, gauges, radios, guns, missiles, electric wiring, pumps, radar, radar receivers, ejection seats, the plexiglas canopy and windshield, the rubber wheels, etc, etc, ad nauseum.
Hundreds of subcontractors have supplied airplane manufacturers since the 1930s.


text:
Where can I learn more about pilots' dreams during G-LOC?
I came across an extremely interesting note from a wiki article on G-LOC (g-force loss of consciousness, or blackout):

Brief but vivid dreams have been reported to follow G-LOC.

Sadly no citation. Where can I read more about this? I did a quick Google search on "pilots dream g-force blackout" but only got 2 relevant hits that didn't tell me much. Some reddit posts described a dream trapped in a maze and a trip to Hawaii, that's about it. They sometimes described it as an out-of-body experience instead of a dream.
Does anyone know where to go to read more? Is this really a common phenomenon?

This was mentioned in the TV episode JetStream - a program about Canadian fighter pilot training, around 20 minutes into episode 1. You may look it up online.

The topic seems pretty "on" to me - a question about an unusual experience that is apparently had by pilots while flying; not only that, but interesting too.

I'm not sure if you've come across this or not, but the paper Recovery from
Gz-lnduced Loss of Consciousness:Psychophysiologic Considerations has some details on this phenomenon. It describes the tests carried out on eight volunteers.

This does not appear to be very common- five 'dreams' were recalled out of 21 acceleration runs. The subjects seem to have indicated that these were definitely 'dreams' as opposed to 'thoughts'. Also, the report notes:

... it is difficult objectively to determine exactly what the subjects experienced during their individual G-LOC episodes (dream or thought) or even state that G-LOC can be equated to a normal sleep mental state (in relation to dream activity)

Another paper Further Support for the  Concept of a  G-LOC Syndrome: A Survey of Military High-Performance Aviators shows that the dream-like state occurs in about 15% of the cases.
The 'dream' state appears to be restricted to <13s and vary from person to person (in some cases, they were not aware that they've lost conciousness).


text:
Can jet fighters be made to be 2 or 3 times bigger than they are today?
I was watching videos on the Su-27 again, and this fighter appears to be a heavy version of jet fighters, which means it has longer range and can carry more weight.
This got me thinking on what is the upper limits of fighters, in terms of size and weight. Is there a fundamental limitation on how big a jet can be and still pull 9+ g's and use very high angle-of-attacks?
Why don't they make them any bigger to get more range (or endurance)? Is there a practical limit? Would it require too much strengthening of the larger wings or something like that?
For simplicity I put a number to it: 2 or 3 times bigger. Can fighter jets be made 2 or 3 times bigger and still have similar performance in maneuverability and thrust/weight ratio?

The question is not whether you can do it but why would you want to.
Fighters, like any other aircraft are designed around mission profiles;  what do you want this aircraft to do, at what speeds, ranges, altitudes, combat radii, endurance, weapon carriage and employment, stealth, etc.  Therefore you would need some sort of justification for building a fighter aircraft this size.  That justification has to be so appealing that it can overcome the negative aspects of building a big fighter.
While these profiles vary considerably between individual aircraft, they are constantly curtailed by the brutal and impartial realities of war.  Gen Stevens pointed out 4 fundamental requirements of a fighter in one of his critiques of the F-35.  These are important to remember as they have been integral in every successful fighter design over the past century of air combat.  Unfortunately, the larger the fighter, the more diametrically opposed to these realities they are and the greater the probability of failure in combat.

ATTACK YOUR ENEMY IN OVERWHELMING NUMBERS
This one is fairly simple to understand.  The greater your force is, the more difficult it is for the enemy to destroy you.  But defense ministries have limited budgets and can only spend so much on new fighters.  So the choice becomes should I buy a few advanced and highly capable fighters or lots of cheap, single purpose aircraft.  Air warfare has favored the latter.  Not only are smaller aircraft cheaper to procure, they are cheaper to operate as they burn less fuel and require less maintenance.  Consequently.  Your nation's pilot pool can get a lot of flight time in them and can become highly proficient in their operation.
SPOT YOUR ENEMY BEFORE HE SPOTS YOU
Which is going to be easier to spot from a distance: a 747 or an F-5?  Which is more likely to have a lower radar or IR signature if both employ LO features?
OUTMANEUVER AND OVERWHELM HIM
By the energy maneuvering equation, fighter design had always favored the following features:

High thrust to weight ratio
Low wing loading
Low specific fuel ratio

And this tends to favor a fighter which is basically a small, fast, nimble gas can with a compact but powerful engine and an effective armament for engaging and killing other fighters.  Not easily achievable with a larger aircraft.
KILL HIM QUICK AND MOVE ON
So now, a large, expensive, easy to spot aircraft manned by low time flight crews is now forced to engage a swarm of nimble, fast turning fighters.  They can quickly turn inside him and get to an optimal firing solution faster and more reliably.  They are also more difficult to engage and shoot using BVR weapons.  Finally OpFor has more experienced and current pilots than the larger fighter.  This now gives the larger, more expensive aircraft a very limited probability of survival.

For those reasons, history does not favor a large fighter unless it is equipped with some kind of mission system giving it a tactical advantage - and even then that is a large obstacle to overcome in terms of cost, strength, and weight.

Carlo, I edited your answer to add a bit of format for organization (Nice answer).  Please review to make sure you like it.

I liked the answers given here. I wanted to add 2 specific examples not mentioned yet, along with an analogy, to help add to what has been posted.
As mentioned, the design of a tactical military jet is a trade-off between effectiveness and ineffectiveness for a given mission. A simple example is that of a light attack bomber. Precision bombing would dictate a stable platform, i.e. low roll rates. The A7-E had a high mounted wing to increase stability in the bombing run.
As a bomber the A7-E was unsurpassed. I was in a bombing derby in 1985 with the newly deployed F-18's. The F-18 had an advanced delivery system, and still I was able to match their CEP bomb for bomb. But the A7-E it was not a fighter. It did not have the thrust-to-weight ratio that a fighter did, and so was at a disadvantage in ACM with an adversary like the F-18.
The A7-E also operated off of carriers based at sea, which can complicate the choice of design. Bombers want long legs to reach inland targets with heavy payloads of bombs, and so fuel efficiency is of particular importance. The A7-E delivered in this respect, and burned around 6-9K of fuel per hour at sea level and military power. This got us up to around 750 mph, which is just below Mach 1. But long legs are not only needed to reach inland targets from the ship, they also serve maintaining real-time surveillance of threats surrounding the carrier out to 200 miles.
When the A7-E was replaced by the F-18, which has the shorter legs of a fighter, the carrier lost the ability to have real-time surveillance of surface threats out to 200 miles. So here you can see the tradeoff affecting the strategic concerns of the battle group.
My last comment concerns design choices for the F-14, which was designed as a long-range interceptor. I knew some Tomcat drivers while stationed aboard the USS Nimitz and they told me an interesting vulnerability that you might not guess. The F-14 interceptor has a variable-swept wing, which is an attempt to mitigate the interceptor design with that of a fighter designed solely for air-to-air engagements. Yet when the Tomcat was in a turn, and G's applied, the wings swept forward making it a barn door in the sky. Easier to spot than let's say the F-16. There was a saying in air-to-air combat something like, "The first one to gain sight wins the fight." So we see here another choice where survivability is sacrificed for the interceptor role.
Finally, consider the evolution of the design of 12 meter class sailing boat. The design of the 12 meter class boat must obey the formula (America's Cup formula):

Where L is waterline length, d is the difference between skin girth and chain girth, S the sail area, and F the freeboard. Designers are free to choose these parameters any way the want as long as when substituted into the formula in comes in at less than 12 meters. Make a sailboat good in all wind conditions, and then in strong winds it will probably be beat by a boat built strictly to win there.

What role would this fighter play?
I think practical answers are that large planes are typically not agile planes and present a larger target for enemy fire.
I'm assuming that this fighter has to perform at least as well as a current front line fighters so you're talking about slinging a lot of mass around at high speeds.  That takes fuel, and fuel adds weight.  And to push more weight through thr sky you need more fuel, which adds more weight...so more fuel.  At some point a long range fighter becomes an attack plane that can also happen to shoot down other planes.
Max weight of an F16-50 is 46,000 lbs.  3x would be pushing 140,000 lbs, or just shy of the MGTOW of a 737-600.  If you stencil "FIGHTER" on to the side of a Lufthansa plane you'll have a giant fighter!  It's perfect...nobody will suspect it of being a deadly long range fighter until it's too late!!  It doesn't need to be fast, it just has to rapidly dispense dozens and dozens air-to-air missiles and a cascade of chaff and flares.  Eventually the defending air force will simply run out of fighters at which point the giant plane just turns around to the nearest airline hub where it quietly blends back in with the rest of the airplanes.

It would play the same role as the Su-27, which is long range air superiority fighter. Last I checked the Su-27 had a range of around 3000 km, which is very very high for a fighter. The role of my idea would be a long range air superiority fighter with even longer range. I like your 3rd paragraph, tho, lol. Whole new meaning to stealth.

Alternatively, the role could be just to have a longer endurance. All else being equal, the fighter with more fuel will outlast the enemy. If two fighters match each other in maneuverability, such that they can only chase each other's tails, the one who runs out of fuel first (or is forced to flee and show his tail) will lose.

I believe a more practical solution is to produce unmanned fighters able of sustained maneuvering beyond 9g.  It's the meat sock in the cockpit that limits a fighter's performance, not the machine.  This article (http://www.popsci.com/ai-pilot-beats-air-combat-expert-in-dogfight) describes the future of air combat.  It's only a matter of time before the tech is perfected.

Yes definitely an unmanned fighter can be less limited, but the question still remains as to how big we can build them. I guess another way of asking it is, can something like the 737 be built strong enough to pull 9 g's? Or do material limitations prevent that?

@acpilot Let the RoE permit BVR engagements and you don't need >9g capability. :p

Then just a huge plane packed with air-air missiles?  At some point you don't need a fighter...you need a 747 with bay doors and hard points!

Why don't they make them any bigger to get more range (or endurance)?
  Is there a practical limit? Would it require too much strengthening of
  the larger wings or something like that?

The SR-71 proved (in a manner of speaking) that there are few practical limits in aircraft design if someone is willing to foot the bill. while that was a high altitude recon plane it still taught the world a lot about extreme aircraft design, so I would say sure what ever you are dreaming can be built.
Whats important is that there is no practical physical limit so to speak but at the same time there is simply no need for such a plane. The key thing to remember is that production planes are built for missions not for exploring physics and engineerings. The fighter role is a short range, nimble plane that carries a light to medium weapons load. Its easier to have a refuel plane loiter near buy and aid the fighter than build a larger, less nimble fighter that can fly farther on its own. The plane you describe might even look close to a B-1 if it were to be built. The fact remains that its more than possible its just not necessary.

The SR-71, while a fast plane, was not a fighter, and it wasn't even very maneuverable. The design load factor limit was just 3.5g's, less than the Cessna 172's so many of us fly (3.8g's).

I kinda agree with your point in the 3rd paragraph, but the SR-71 is a bad example to compare to. It was designed for stealth and speed, not maneuverability. I would have a very bad feeling if it tried to pull 9 g's even at subsonic speeds.

@RonBeyer It was derived from YF-12, was it not?  Foxbat MiG 25 was a fighter, but it wasn't built for a turning fight; it was built as a high speed, high altitude interceptor.

I believe the SR-71 was used as an example of what can be done, not of a large fighter. All sorts of engineering challenges were overcome to build the plane because someone was willing to pay for it and there was a need for its design mission. Same can be said for a much larger fighter. if there's a need and a budget, it can be done.

Can jet fighters be made to be 2 or 3 times bigger than they are
  today?

Yes.  Consider the Tupolev Tu-28 (or Tu-128) "Fiddler" interceptor.  Also the Lockheed YF-12A.

Did those aircraft have similar maneuverability and thrust to weight ratio than the Su-27? If so, I'm quite curious to read more about the YF12 maneuverability and thus you may provide further reading.

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

Tu-128 is interesting, but the wikipedia page says the g-limit is "+2.5". Makes me think it is not close to 9. The note about "fighter" even says it was not intended for fighter-to-fighter combat, only intercepting bombers. YF-12A, can't even find a g-limit for it but I would be very surprised if it got anywhere need 9 g's. And again, it's only an interceptor. If they did envision fighter versions of them, I would guess it was during the era they thought missiles would do all the work and you'd never slow down to subsonic dogfight (both wrong).

@DrZ214 it was a fighter, albeit one with a different role from what you may have in mind. Remember too that many fighters of the era had similar g limits.

@slantalpha it does answer the question, even if you don't like the answer.

The answer was improved since the second comment ("This does not provide and answer...") was posted.


text:
Why would a commercal flight circle while enroute?
While selecting random flights to learn about FlightRadar24, I came across AC7311 flying from Toronto, ON (CYYZ) to Indianapolis, IN (KIND), and saw that it flew a couple of circles southwest of Lima, OH (near KAOH).

What are some of the reasons Air Traffic Control would direct the pilots to do that?

that point is about 100 miles out from the airport, (at least on google maps' measure distance it is,

As others have noted, this is called a holding pattern, which is used to delay an aircraft while in the air. Of course, this takes time and fuel, so the system works to prevent this from being necessary.
The primary use of holding patterns is by ATC to delay arrivals. This could for traffic reasons, to delay the aircraft until it can join the line of arriving aircraft. It could also be for facility reasons. Bad weather can reduce the rate at which an airport can accept aircraft, meaning some will have to enter a hold. An airport can also be closed for weather, or for another reason, in which case all aircraft must enter a hold, or land at an alternate airport.
Pilots sometimes need to use holding patterns as well. If they have an issue with the plane, they may need time to follow their checklists and diagnose the problem. ATC will give them a place to hold for this time.
Standard arrival procedures generally have holding points published for these cases.  When a holding pattern isn't defined, one can be created. The aircraft will fly for a standard time before turning around, and repeating as necessary.
A better FlightRadar example is perhaps the following image, showing three aircraft in the stack and one leaving it

Source

If I may ask -- which of the Heathrow stacks is depicted here?

@UnrecognizedFallingObject Based on the image here it appears to be Ockham.

It's called Holding Pattern.
ATC can direct a plane to enter the pattern in order to ensure separation between landings:
two planes might reach the airport almost at the same time, the first might be directed to land, the second will have to wait.

They would have them hold that far from their destination? I always envisioned holding patterns to be much closer to the destination airport.

far is relative. Remember that you have to proceed to the approach and for large aircraft/airport it usually is several nautical miles long.

@FreeMan Yes. Airliners fly quite fast (around Mach 0.85,) so their turn radius is not small and their descent/approach pattern is also quite long. Furthermore, they want to keep holding aircraft somewhat distant from the airport just for separation purposes with arriving and departing aircraft. Also, several different holding patterns may all be in use at once, so they need sufficient separation from each other as well. For example, when a thunderstorm passes over Atlanta International, a lot of arriving flights can back up very quickly and they all have to go somewhere.

They don't typically fly at Mach 0.8 in a holding pattern, though :p

This maneuver of doing a 360° turn is issued by ATC. There are two reasons for this:

to increase spacing between your flight and another airplane
it was one or more circles in a holding pattern

By looking at the map, it appears that what your interested flight did was a holding pattern, because it is not too far to the airport.

It's a holding pattern. Note that it's actually not a 360 degree turn. It's 2 180 degree turns with straight sections in between them, as is normal for a holding pattern. 360s are just that - 360 degree turns with no straight section and you just roll back out at the same point that you entered.


text:
What would it take to have a new large civil aircraft manufacturer?
What would it take to have a new large civil aircraft manufacturer besides Boeing and Airbus, that is not state sponsored (C919, MC-21)
In other words, what are the chances a new company would break into the market? What would it take?
If you look at the US past aircrafts, there is an abundance of companies producing large civil aircraft: Boeing, Convair, Douglas, Lockheed, Martin. Then there were the French, the British and the Russian manufacturers. Now there is Boeing and Airbus. Embraer and Bombardier are producing rather small planes for today's standards, even if they are impressive compared to historic planes.
The regional jet market is a lot more competitive, with the CRJs and ERJs under pressure from MRJ, SJ-100 and ARJ. Could one of those companies successfully grow into Boeing and Airbus market?
What are the chances a start-up, with no state money, will come to market with a new 100+ seats plane in the next 20-30 years? Could a SpaceX type of company be successful in the airplane manufacturing industry?

Question: How do you make a small fortune in the airline business? Answer: You start with a large fortune.

You are comparing aircraft manufacturers and SpaceX. Without NASA money there wouldn't be any SpaceX. Thus, the whole question is based on a false premise. The aerospace industry is, in general, subsidized by governments.

The one company that I've always expected to jump into this business is GE. I wonder why they don't. They have the deep pockets & the aviation connections from their engines division. Besides they are already into heavy industries that would give them the talent & facilities for constructing 100+ PAX aircraft.

@curious_cat why would they throw away a few billion at designing and marketing an aircraft that would compete with other aircraft they already make a few billion on supplying the engines?

@jwenting Which aircraft does GE make that a 100+ seat model would compete with? To me it sounded forward integration.

@curious_cat Also, GE Aviation doesn't just make the engines. They also make avionics, etc. One of my friends designs avionics for them. IIRC, he's been working mostly on systems for the 737.

@curious_cat that's the point, they DO NOT make an aircraft because they'd be in competition with a large saturated market.

It would take lots of money.

Too much to competition with especially trying to develop a new aircraft against companies like Boeing or airbus. Again  LOTS OF MONEY

What are the chances a start-up, with no state money, will come to market with a new 100+ seats plane in the next 20-30 years?

If you count Bombardier, the chances are roughly 100%. The Bombardier C-Series, which will seat 140 (160 max,) is scheduled to enter service within this year (2015.) However, Bombardier is hardly a 'start-up' in the normal sense.

Bombardier C-Series Mock-Up (Source: Wikipedia)
For a real start-up (e.g. a SpaceX-style company,) the chances will be much more slim. It takes billions of dollars worth of investments to design a modern commercial airliner before you will ever see the first dime of revenue. Start-ups generally won't have access to that kind of money until they have had time to mature and developed a steady revenue stream from other successful products (which is how Bombardier is doing it.)

Could a SpaceX type of company be successful in the airplane manufacturing industry?

That depends greatly on the type of airplanes in question. Undoubtedly, this is possible in the light general aviation market. Several such companies have come and gone over the years. One interesting start-up that is planning to enter production soon is Terrafugia, a start-up from MIT, which is building aircraft that fold up into street-legal cars that can just drive away from the airport and park in your garage. However, the larger airliner market is a completely different ballgame, involving development costs that are orders of magnitude higher. Start-ups have a difficult time getting started in markets with such huge barriers to entry.

except that Bombardier is hardly a startup AND is heavily state backed (or at least used to be, not sure about now that they're well established and have been a successful business for decades).

Reading this now, a new answer becomes obvious: Do as SpaceX, hire a bunch of good engineers highly frustrated by their beancounter management, and you can start a very successful airliner company even (different from SpaceX) without state aid. But you still need 5 years and 5-8 billion USD until the first delivery.

This question is much more about economics than about aviation. What would it take to have a new large civil aircraft manufacturer? It is a question of economics, and the analysis is essentially indifferent to the industry, per se.
No surprise that the regional market is more competitive, in the sense that there are more aircraft manufacturers. Generally, as aircraft increase in size they become more difficult to produce while the market for them grows smaller.

What are the chances a start-up, with no state money, will come to market with a new 100+ seats plane in the next 20-30 years?

The Bombardier C-Series/Airbus A220 history suggests that it will not happen profitably any time soon.  Problems include:

Trade wars.  The simple fact that Boeing can create the impression that there will be tariffs is a big threat.
Price wars.  Airbus lowered their prices to reduce the value of the C-series program, allowing Airbus to get a controlling stake for free.
More money can be had from selling your manufacturing program than from selling aircraft.  Before Airbus was involved, Bombardier was losing money and receiving bailouts.  Airbus paid Bombardier for an extra 25% of the program.

All this happened when Bombardier was an established company with state support.  A startup would fare worse.
In short, only someone willing to lose a vast sum of money could do it.

“only someone willing to lose a vast sum of money could do it” is, to a large extent, how SpaceX came to be, as a high risk investment by someone who already had a different company producing fairly reliable income.


text:
Is a Control Law Degradation in Airbus Planes displayed on the ECAM?
I would like to know if there is any warning displayed on the ECAM, when control laws get degraded from, let' say normal to direct law, other than the small text on the PFD.
Also if there are any sounds played in the cockpit.
This is because I am at the moment intensively researching myself on XL Airways Germany Flight 888T.
Have there been any software updates, that tell the pilot if the laws are getting degraded since then?
Maybe even because of that crash.

See this for the various ECAM messages, specifically ATA 27

Since you are trying to understand how the crew is aware of law reconfiguration, it's important to look at this event in its context, why it does happen and what are the consequences for the control of the aircraft. You may skip this overview and go to the direct answer to your question which is in the last section of this post.

Flight control system and computers
The crew and the FMGS (autopilot) send commands to redundant computers:

2x ELAC (elevator aileron computer) for elevators/THS and ailerons,
3x SEC (spoilers elevator computer) for elevators/THS and spoilers,
2x FAC (flight augmentation computer) for rudder,

and computers send commands to actuators.

Source.
When the system is fully functional, all computers work using a normal law and it provides protection against:

excessive pitch and roll attitude,
high-angle-of-attack,
high-speed and low energy,
excessive load factor.

This can be done by receiving aircraft dynamic data from multiple sensors visible in the diagram.
If a flight control computer fails (or stop receiving required data), the crew receives an indication on the overhead panel:

Source.
When one or several of those computers fails, the role of the remaining ones are redistributed.

Reconfiguration of the computers
The failure of an element of the flight control system needs that other available elements compensate for the missing computer or peripheral. This reconfiguration allows available computers to control an additional axis using other laws. However departing from the normal law for an axis means losing one or more of the protections available under the normal law.
Degraded laws are:

alternate law (two levels: With and without reduced protections),
direct law (no protection),
mechanical (mechanical linkage between controls and control surfaces).

The mechanical mode is a backup mode to control the tail surfaces from the pedals and the pitch trim wheels in case something prevent actuators to work (e.g. loss of all electric power sources before the RAT is deployed).

Source: Airbus A320 FCOM
Alternate and direct control laws modify the relation between the stick movement and the flight control movement (linearity and amplitude).
Effect of pitch axis alternate law in different phases of flight
After the flight control system has transitioned to an alternate law, some protection may be available, but law may also transition again according to events representative of the different phases of the flight. Here is what happens around alternate law on pitch axis, which a change to direct law as the landing gear is extended:

Source: Airbus A320 FCOM
A very degraded flight control system: Direct law
While alternate law on a given axis still maintains (under some conditions) some level of flight envelope protection, this protection is lost when degrading to direct law. According to the axis, the direct law impacts are:

Pitch: The stick commands linearly elevators movement, automatic pitch trim is unavailable, all pitch protections are lost as well as alpha-floor.
Roll: The stick commands linearly surface movement, but overall amplitude is different according to slats/flaps status, only ailerons and spoilers 4/5 are used (all spoilers if ailerons fail).
Yaw: Mechanical control using the pedals, yaw dumping and turn coordination unavailable.

Permanent indications on PFD
Green $\sf \scriptstyle \color {green} {\text {=}}$ indicate active protections: Bank angle ± 67°, pitch angle at 15° nose down and 30° nose up. On the speed scale, speed limits for alpha protection.
Loss of protections after the degradation from normal law is indicated by amber $\sf \color {orange} {\text {x}}$.

Source: Airbus A320 FCOM
When the direct law is in effect for pitch the amber warning $\sf \scriptstyle \color {orange} {\text {USE MAN PITCH TRIM}}$ is displayed below the lines of the flight mode annunciator.
In addition if the law transitions to mechanical backup (which is supposed to be a transitory state), a red $\sf \scriptstyle \color {red} {\text {MAN PITCH TRIM ONLY}}$ replaces the amber indication.
General alerts
Audio chimes are triggered at the time of a component failure, when the mode changes. Alerts also activate either of the overall alerting lights (master warning and/or master caution).
ECAM messages
Failures and law activation trigger messages on the ECAM, with suggestions for crew actions.

Source.
However, and this was noted by investigators on AF447 accident, ECAM messages can be shown or hidden in the scrollable list according to messages priorities, due to the high number of messages that may appear in a short period of time during such sequences of events:

Source.

Thank you a lot! I love diving into the complex Airbus systems.


text:
Could the crew of XL Airways Germany Flight 888T not hear important chimes, due to the stall warning?
I am intensively researching on XL Airways Germany Flight 888T at the moment.
I wrote and researched hours and hours. Now I would like to know something about the ECAM and the Airbus Stall warning:
The crew of 888T started a stall test with faulty AoA-Sensors. They wanted to test the stall protection system. At a point, the Airbus stall warning was played. A few seconds later, the control laws degraded to Direct Law. The crew missed this degradation and I just want to find out why.
I found out, a control law degradation is a level 2 ECAM warning. That means a single chime will be played. Can someone confirm this?
I also heard, the stall warning suppresses all other sounds, because a stall has priority over all other things. But I don't know if this also applies for the ECAM Sounds. Does anybody know if a stall warning really suppresses every sound?

I just want to find out why. have you read the investigation report? how do you hope to find out anything new? do you have access to information that they did not have?

@Federico Yes, I read it, but I want to find out if Airbus could have prevented the crash with a better situational awareness, even though they were so many factors that made the plane crash. If you are interested: https://docs.google.com/document/d/1y7ry2WxkVDKHRtMsrkmO3eRwtdn7zAIHFQsPIXAiIR0/edit?usp=sharing

The fact that audio warnings may be ignored under stress is well known. Read the conclusions of BEA report on AF447, where the stall alarm was ignored. You cannot say, 1/ "just follow what avionics says it's easy", and 2/ "if avionics has failed, it is obvious you can't trust it". Unfortunately the faulty system don't tell the crew it has failed. The most obvious solution is longer training paid by more expensive tickets. The customers don't want it.

@mins I did not know that control laws degrade to alternate, in case of a stall. Thank you. Can you, by any chance confirm that the degradation is a Level 2 ECAM Caution? Please post an answer in that case to get the reps.

When the stall has developed the aircraft is no longer in normal control law for any axis (see more in FCTM at OP-020. P 14):
As soon as the aircraft is outside the flight envelope, the control laws transition to abnormal attitude which is the combination of:

Roll direct
Pitch alternate
Yaw alternate.

This is something between alternate and direct, and is intended to allow the crew to recover from the abnormal attitude and reenter the flight envelope without being prevented by some protections.
A stall (an excessive angle of attack) is one of the conditions to switch to the abnormal attitude laws. You may read this article too, while on a simulation site, it is very well written. There is also a presentation on SKYbrary at Eurocontrol.

When the stall warning aural warning is active (voiced "Stall" and cricket), the crew is expected to know they are in abnormal law.
Reconfiguration in abnormal attitude law is indicated by different clues on the PFD (e.g. $\alpha$ prot and $\alpha$ max indication removed from the speed strip, and green = replaced by amber crosses).

Source

Source
and the master warnings are activated. The aural alert is a single chime, (level 2 failure confirmed in this report, page 25). The reconfiguration is also indicated on the ECAM, but the indication can be moved towards the bottom of the list if messages with higher priority arrive.
The law is reconfigured as direct when the landing gears are lowered.

Regarding the aural warnings associated with stall and law reconfiguration, they are audible at the same time, or are intercalated, but none cancels the other, as it was heard for AF447 and reported in this article (C-chord is the single chime):

In sequence the alarms were sounding PULL UP, C-chord, STALL, C-chord,
  PULL UP, PRIORITY RIGHT.

But did they actually get a stall warning on XL888T? The cause was that their AoA vanes were frozen stuck and a stall warning is based on those, no?

@JanHudec: From BEA report: "At 15 h 45 min 05, the aeroplane was at 2,910 ft altitude and a speed of 99 kt. Pitch angle was 18.6 degrees. The stall warning sounded.". The crew received four stall warnings. One of the three sensors was still operative. The last stall warning stopped at 15 h 46 min 00. Seven seconds later the recorder stopped. See p.115 for the stall warning (and master warning) occurrences on the FDR.


text:
Do modern aircraft have the functionality for the pilots to look behind the aircraft?
Basically like the functionality of a rear-view mirror in a car. It's made known that pilots have all the information, hence they might not need the feature, but my question is to know if such a feature exists in modern commercial airlines.

Civilian or military? Related: Why don't airliners have any mirrors or cameras?

Why would they need to? Most modern aircraft usually don't "power-back", there are a few exceptions but usually those aircraft have good rearward visibility.

Worth noting that, in Instrument Conditions, the pilot doesn't necessarily have the ability to look in front of the aircraft, either....

Interestingly, the A380 has a camera on the vertical stabilizer, but opposed to what you might expect, it faces forward.

@JonStory: Exactly what I edited my question to.

Some do, some don't.
It depends by the type of the aircraft and by the need to see behind the aircraft itself.
Just as an example, this is a render of a Boeing F/A-18E "Super-Hornet" cockpit:

On the canopy's frame are located three adjustable mirrors (like the one on the windshield of a car) the pilot can use to see what's happening behind him.
As already stated in comments, aircraft usually don't "power-back" and there is no overtaking like in cars (where cars from behind can overtake you just from left or right), so usually there is no need of mirrors inside a cockpit.
In a military aircraft, mirrors increase your situational awareness, but

with a wide Field-of-View (FoV), details or a distant aircraft or missile cannot be seen
with a narrow FoV, a smaller portion of the sky is visible

Just for reference, the F-16 or the F-22, both featuring a single piece canopy, don't have rear view mirrors. The pilot must turn the head as much as he/she can and look behind.

I've been told before that the mirrors are primarily only for spotting if you're leaving con-trails behind you, which would be a dangerous giveaway for a fighter jet.   Their field of view is too small for situational awareness.  I'm not sure how true it is, but it sounds reasonable.

@abelenky Well, at least to me, it sounds not so much reasonable, because then I should ask... why three mirrors? Beside this, fighter pilots usually fly in formation. A wingman could easily spot contrails (and other threats, by the way) for the leader or vice versa.

Almost all tow planes have rear view mirrors mounted in them so they can see the glider and the tow rope.
You can see mirrors mounted on the cowlings of these Piper Pawnee tow planes.

There is one example that I can think of.
The Norhrop Grumman AN / AAQ-37 Distributed Aperture System,
Developed for the F-35 airplane, this sensor suite for the aircraft consists of six visible and infrared cameras placed at points around the fuselage.  The images which these cameras create are fed to a central processor, which not only uses the information for automated functions such as missile launch warnings, but also creates a 360deg picture of the space around the jet projected directly onto the visor of the pilot's half million dollar helmet.
http://www.northropgrumman.com/capabilities/anaaq37f35/pages/default.aspx


text:
What does the area around the fan blades in the engine nacelle contain?
I was intrigued to find this picture below. There was so much space around the engine. So what is inside of that extra space? Or is it just a hollow casing?

Image Source
Details about the picture as provided in the image source:

[Last Year] A Southwest Airlines Co. flight landed safely Saturday
  morning following a major malfunction of one of its two engines during
  a flight across the Southeastern U.S.
The Boeing Co. 737-700 was flying from New Orleans to Orlando early
  Saturday operating as flight 3472 when it suffered the failure,
  causing the jet to quickly divert to Pensacola International Airport
  in northern Florida at 9:40 a.m. CDT, according to a statement from
  the airline.
A Southwest spokesman said its initial reports indicated no injuries
  were reported among the 99 passengers and five crew aboard.

"major malfunction" seems like it is downplaying that a little.  I'd go with a minor unplanned dissasembly myself.

Yes a bit of the engine called the nacelle is missing. This is an aerodynamic structure that slows the air down before it enters the compressor, the pointy bit in your photo. In the drawing underneath it states Inlet Cowl. It has some thickness, because it requires a certain nose shape for its function. Mainly a hollow casing indeed, but with a high tech shape.

Not "Just a hollow case".  This is also where the anti-ice for the engine intake is.  Without that, you're essentially limited to flight in clear skies -- which drastically curtails commercial flight.

I've slightly changed the phrasing. This video seems to indicate that the anti-icing is mainly a hot air conductor from the compressor to the inlet: a pipe, which is also a hollow casing :)

In the image the front part of the engine nacelle (the inlet region) is missing. This region is usually hollow, with the equipment (like control unit) and tubing coming after the separator wall visible in the photo. The main purpose of the missing region is aerodynamic.
In the image below, you can see the equipment and various other items covered by the cowlings (which are open). The separator wall can be seen in the far end of the engine, near the fan blades. Though this is not the 737-700's engine, the design is similar.

Engine with cowlings open; image from kativ.eu

I imagine there will be some ducting and accomodation for the anti-icing system?

@ROIMaison You're right... That region hosts the ducting for anti icing system...

In fact, look at the stuff behind the separator wall.  See how much of it is below the fan?  That stuff is moved to the side on the 737 engines to improve ground clearance.  That is why they are oddly shaped.

What is missing from your photo is the engine inlet. As someone stated, the inlet does contain some anti icing components, likely a swirl nozzle for a commercial application.
The engine inlet structure is composed of an inner composite barrel, and outer composite barrel, a forward bulkhead, an aft bulkhead, and the inlet lipskin.
The inlet attaches to the forward mounting flange on the fan case (the forward most flange).
Anti-ice ducting from the core interfaces with the inlet, which has another tube (most commonly with some sort of flexible component for thermal expansion) that spans the distance between the forward and aft bulkhead (which appears to be hanging back from about the 11 o'clock position on the engine from what is left of the aft inlet bulkhead).
This then interfaces on the forward side with the anti-ice distribution system in the area inside the lipskin, known as the "D-Duct". The area between the bulkhead of the inlet is indeed empty.
The inlet is specially designed for acoustic attenuation.
The components you are expecting to see are contained in two places: the engine core and the engine fan compartment. Depending on the engine, these two locations house the electrical, fuel, hydraulic, pneumatic, and oil components of the engine. These systems are commonly referred to as the "EBU" or engine build unit or engine build up.
Source: I am an EBU Systems Design Engineer

The area immediately at the end of the fan blades is filled with a fairing compound. It is smoothed to within a few thousandths of an inch (slightly) bigger than the diameter of the fan. This compound sits in a depressed are built into the support ring for the cowl. It keeps the air flowing smoothly through the fan blades. Between the support ring and the cowl there are some control actuators and support structure. Also the bleed air ducts for engine anti ice may be routed there. It will all depend on the model and manufacturer design considerations. Modern cowls are actually designed to provide forward lift and airflow acceleration. First used on WW2 radial engine aircraft.

Welcome to Av.se!

Above all, the clearance in the engine pod is to facilitate routine maintenance and quick visual assessment during turnaround cycle of operations. Further, this greatly helps in efficient engine replacement cutting down on 'Downtime' !
This is one of the factors, I feel.

Above all, the clearance around the engine behind the fan section is to provide a channel for smooth by-pass air flow. In the last picture you can see that this particular design has a separate inner cowling around the non-fan portion of the engine, as opposed to building the by-pass channel into the rear section of the nacelle that in the IPB example also houses the thrust reversers.  The easily raised cowlings provides very fast access for maintenance purposes; but, turn inspections for modern engines is little more than a walk-around looking for leaks or external damage.


text:
Why does the A320 transponder's CLR button do two different things, depending on how many digits were entered?
I have to admit I've only flown in simulators but everyone of them implements the "CLR" button of the transponder in an A320 (maybe also others) the following way:

It removes the fourth digit only if present
Otherwise (meaning at most three digits defined) it clears the complete code

Why is it that way and why doesn't it either clear each digit individually, or all digits every time?

It might be good to know what kind of flight simulators you have experience with because not all are intended to emulate each function of the real aircraft.

I've tested it myself on the FS Lab A320, but seen in on the Flightfactor and MS2020 A320s.

Well hopefully someone with A320 experience will chime in here, but I can tell you from experience that not everything in MS functions like it does in the real airplane.  Consumer PC simulators have gotten pretty darn good, but they are after all really just games so I wouldn't put too much stock on them being accurate...

Not a pilot, but it seems pushing CLR shortly clears the last digit entered, pushing 2s clears all digits. At least on a  Gables ATC-TCAS panel.

It may be linked to the fact that when you enter the last digit the previous code is removed, while if you enter only 3 digits or less, the previous code is still active.


text:
What documents do I need to take a German airplane outside of Schengen but still within the EU?
What kind of documents are required for customs or police if one flies out of Schengen (in this case a German-registered aircraft) and lands in a non-Schengen state within the EU?

Welcome to Aviation.SE! Questions about customs or police (I guess you mean immigration?) should be asked on Travel.SE. You can ask about relevant aviation regulations here.

I mean for the airplane.

OK, but then Schengen has nothing to do with it. It is only about immigration. What might be more relevant is whether or not the country you fly to is an EASA member state (as far as I know, all EU members are also EASA members).

I see, so an "airport of entry" is only about customs for people, not for "legally taking the plane" to a country?


text:
How do I get access to images and diagrams during the private pilot knowledge test?
I am not sure if the testing center is online or paper based. However, on the private pilot knowledge test, how would one manage the questions related to density altitude, ground roll distance etc., when the images are on the computer and one has to use a paper/ ruler to get the right answer.
Are we allowed to take the print outs from the following website to the exam?
https://www.faa.gov/training_testing/testing/supplements/

The testing center provides a physical, paper booklet with all the diagrams, pictures, charts etc. needed for the questions. You can't write on it (well, you can but you shouldn't!) however you can certainly use a ruler or paper edge as you mentioned. The test itself is online.

You are only allowed to take plotters, certain calculators, and E6-Bs into the test. Details are at p 3-5 and 3-6 Order 8080.6H There are some restrictions on the types of calculator. The aviation-specific ones sold by places like Sporty’s and ASA are OK. Fancy ones that can store programs are not.

b. Test Materials. Applicants may use scales, straightedges,
protractors, plotters, navigation computers, blank logsheets, holding
pattern entry aids, and electronic or mechanical calculators that are
directly related to the test.
c. Manufacturers’ Aids. Manufacturers’ permanently inscribed
instructions on the front and back of such aids (e.g., formulas,
conversions, regulations, signals, weather data, holding pattern
diagrams, frequencies, Weight and Balance (W&B) formulas, and air
traffic control (ATC) procedures) are permissible.


text:
Do study aids for the private pilot knowledge test include the actual questions that will be in the test?
I have been working with the Sporty's course for a while, and doing their FAA Test preps. What are the chances that the questions I am learning on are the same on the actual test?
Are the questions on FAA Knowledge test different than what studied on Sporty's or any other Online Course?

Multiple guess test questions are diabolical if done properly.  You have two obviously wrong answers, and two seemingly correct answers, that require you to really know the material to parse them and decide which one is the right one. If you don't know the material well enough, it's only a 50/50 chance you'll get it right.

The official answer is in an FAA FAQ:

The FAA does not publish actual knowledge test questions, in part
because at least two independent studies indicate that publication of
active questions could negatively affect learning and understanding,
as well as undermine the validity of the knowledge test as an
assessment tool.
The agency does provide sample knowledge tests on the
FAA website. The questions in these sample tests are intended to help
applicants understand the scope and type of knowledge that will be
tested to qualify for the target certificate or rating. The goal is
for applicants to devote their efforts to mastering the fundamental
aeronautical knowledge necessary for safe operations in the National
Airspace System (NAS) rather than to memorizing specific questions and
answers.
The questions that you may have seen and studied in
commercially-available materials have been developed by test
preparation providers for similar reasons – that is, to enable
applicants to study concepts and practice calculations specified in
the 14 CFR part 61 “aeronautical knowledge” requirements for each
airman certificate or rating. These are not, and should not be
represented to be, “real” questions.
In some cases, unscrupulous test
preparation providers have sought to obtain actual test questions by
overtly or otherwise encouraging knowledge test takers to share
information about actual questions and possible answers immediately
after taking the knowledge test. The FAA has taken action against such
companies. You should also be aware that an applicant’s participation
in such practices could be a violation of 14 CFR part 61, section
61.37

The unofficial answer is that third-party study aid questions are extremely - or suspiciously - close to the real test questions. At least one provider even guarantees that they're the same. Clearly that's only possible because providers are getting the questions from somewhere, but I imagine they don't want to talk much about it.

The FAA had the entire question bank available for public view once, back in the 90s or so. Something like 500 or 1000.  I guess that ended some time ago.

The questions are the same, though the answers identified as being "correct" may be different in a few rare instances.

uh oh...thats scary. Does that mean, the answers we get from Sports/ or study aids are wrong sometimes?

Even more scary: the answers from the FAA might be wrong sometimes.